Flue gas desulfurization systems and methods of use

ABSTRACT

Flue gas desulfurization processes and systems that utilize ammonia as a reactant, and in which any hydrogen sulfide and/or mercaptans within the ammonia are separated during the desulfurization process so as to prevent their release into the atmosphere. The process and system entail absorbing acidic gases from a flue gas with a scrubbing media containing ammonium sulfate to produce a stream of scrubbed flue gas, collecting the scrubbing media containing the absorbed acidic gases, injecting into the collected scrubbing media a source of ammonia that is laden with hydrogen sulfide and/or mercaptans so that the injected ammonia is absorbed into and reacted with the collected scrubbing media, stripping the hydrogen sulfide and/or mercaptans from the collected scrubbing media by causing the hydrogen sulfide and/or mercaptans to exit the collected scrubbing media as stripped gases, and collecting the stripped gases without allowing the stripped gases to enter the stream of scrubbed flue gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 12/725,766, filed Mar. 17, 2010, which is adivision patent application of U.S. patent application Ser. No.12/109,441, filed Apr. 25, 2008 (issued as U.S. Pat. No. 7,771,685). Thecontents of these prior applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention generally relates to processes and systems forremoving acidic gases from flue gases produced by power generation,industrial, and other facilities. More particularly, this invention isdirected to flue gas desulfurization (FGD) processes and systems thatutilize ammonia to capture sulfur dioxide and produce ammonium sulfate,with the further capability of preventing or at least reducing therelease of hydrogen sulfide and/or mercaptans into a scrubbed flue gasstream produced by such processes and systems.

Acidic gases, including sulfur dioxide (SO₂), hydrogen chloride (HCl)and hydrogen fluoride (HF), are known to be hazardous to theenvironment, and as a result their emission into the atmosphere isclosely regulated by clean air statutes. For the removal of acidic gasesfrom flue gases produced by utility and industrial plants, gas-liquidscrubbers (contactors, absorbers, etc.), are widely employed. Scrubbersgenerally employ a liquid-containing media that is brought into intimatecontact with a flue gas to remove acidic gases by absorption. Theprocess by which acidic gases are removed from flue gases in this manneris generally referred to as wet flue gas desulfurization (wet FGD).

The cleansing action produced by scrubbers is generally derived from thepassage of a flue gas through a tower cocurrently or countercurrently toa descending liquid medium. Calcium-based slurries, sodium-basedsolutions and ammonia-based solutions are typical alkaline scrubbingmedia used in flue gas scrubbing operations. The cleansed gases areallowed to exit the tower, typically passing through a mist eliminatorto atmosphere. The liquid medium and its absorbed gases are collected ina tank, typically at the bottom of the tower, where the absorbed gasesare reacted to form byproducts that are useful or at least not harmfulto the environment. While scrubbers utilizing calcium-based slurriesgenerally perform satisfactorily, their operation results in theproduction of large quantities of wastes or gypsum, the latter havingonly nominal commercial value.

In contrast, ammonia-based scrubbing processes have been used to producea more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos.4,690,807 and 5,362,458, each of which are assigned to the assignee ofthe present invention and incorporated herein by reference. In theseprocesses (also known as ammonium sulfate flue gas desulfurization, orAS FGD), the scrubbing solution is accumulated in a tank where theabsorbed sulfur dioxide reacts with dissolved ammonia (NH₃) to formammonium sulfite ((NH₄)₂SO₃) and ammonium bisulfite (NH₄HSO₃), which areoxidized in the presence of sufficient oxygen to form ammonium sulfate((NH₄)₂SO₄) and ammonium bisulfate (NH₄HSO₄), the latter of which reactswith ammonia to form additional ammonium sulfate. A portion of theammonium sulfate solution and/or ammonium sulfate crystals that form inthe solution can then be drawn off to yield the desired byproduct ofthis reaction.

Ammonia produced from sour water in refinery, tar sands and othersimilar applications typically contains low concentrations of hydrogensulfide (H₂S) and mercaptans (or thiols, which are compounds thatcontain the functional group composed of a sulfur atom and a hydrogenatom (—SH)). When the ammonia is used in an ammonium sulfate FGD systemto capture sulfur dioxide, the hydrogen sulfide and mercaptans arestripped from the ammonia because of their volatility in ammoniumsulfate, and thereafter become entrained with the flue gas. Scrubbedflue gases that exit the FGD system contain the entrained hydrogensulfide and mercaptans which, in addition to being air pollutants, cancontribute an undesirable odor to the FGD emissions.

Current technologies employed to purify ammonia and separate it fromhydrogen sulfide and mercaptans are expensive capital-intensiveprocesses, and consume considerable amounts of energy. As such, it wouldbe desirable if an improved process were available that was capable ofgreatly reducing the amounts of hydrogen sulfide and mercaptans releasedfrom AS FGD systems, while also avoiding the disadvantages of prior artprocesses.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides flue gas desulfurization processes andsystems that utilize ammonia as a reactant, and in which any hydrogensulfide and/or mercaptans within the ammonia are separated during thedesulfurization process so as to prevent their release into theatmosphere.

According to one aspect of the invention, a flue gas desulfurizationprocess includes absorbing acidic gases from a flue gas with a scrubbingmedia containing ammonium sulfate to produce a stream of scrubbed fluegas, collecting the scrubbing media containing the absorbed acidicgases, injecting into the collected scrubbing media a source of ammoniathat is laden with hydrogen sulfide and/or mercaptans so that theinjected ammonia is absorbed into and reacted with the collectedscrubbing media, stripping the hydrogen sulfide and/or mercaptans fromthe collected scrubbing media by causing the hydrogen sulfide and/ormercaptans to exit the collected scrubbing media as stripped gases, andcollecting the stripped gases without allowing the stripped gases toenter the stream of scrubbed flue gas.

According to another aspect of the invention, a flue gas desulfurizationsystem includes means for absorbing acidic gases from a flue gas with ascrubbing media containing ammonium sulfate to produce a stream ofscrubbed flue gas, means for collecting the scrubbing media containingthe absorbed acidic gases, means for injecting into the collectedscrubbing media a source of ammonia that is laden with hydrogen sulfideand/or mercaptans so that the injected ammonia is absorbed into andreacted with the collected scrubbing media, means for stripping thehydrogen sulfide and/or mercaptans from the collected scrubbing media bycausing the hydrogen sulfide and/or mercaptans to exit the collectedscrubbing media as stripped gases, and means for collecting the strippedgases without allowing the stripped gases to enter the stream ofscrubbed flue gas.

In flue gas desulfurization processes and systems of this invention, thescrubbing media can be collected in a tank, and the hydrogen sulfideand/or mercaptans can be stripped from the scrubbing media in the tankor in a separate vessel fluidically connected to the tank. In eithercase, it can be seen that a significant advantage of this invention isthat ammonia containing undesired levels of hydrogen sulfide andmercaptans can be used in a desulfurization process, but with a greatlyreduced risk of releasing gaseous hydrogen sulfide and mercaptans intothe atmosphere along with the scrubbed flue gas. As such, the presentinvention is capable of avoiding disadvantages associated with currenttechnologies used to purify ammonia and separate it from hydrogensulfide and mercaptans prior to its use in an ammonium sulfate-basedflue gas desulfurization process.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a flue gas scrubbing apparatusconfigured for in situ removal of hydrogen sulfide and mercaptans inaccordance with an embodiment of this invention.

FIG. 2 is a schematic representation of a flue gas scrubbing apparatusequipped with a dedicated stripping tank for removal of hydrogen sulfideand mercaptans in accordance with another embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 represent two embodiments of ammonia-based FGD systems 10and 100 that allow the use of ammonia containing hydrogen sulfide and/ormercaptans. The systems 10 and 100 employ a relatively lowcost processfor reducing the release of hydrogen sulfide and mercaptans byseparating and capturing these pollutants after they are stripped fromthe contact media used in the AS FGD process. The process can beintegrated into an otherwise conventional AS FGD system and operationthrough relatively uncomplicated modifications to the AS FGD system,incurring minimal capital investment and energy consumption. FIGS. 1 and2 are drawn for purposes of clarity when viewed in combination with thefollowing description, and therefore are not necessarily to scale.

The invention utilizes the fact that ammonia is soluble in water andacidic solutions, while the solubilities of hydrogen sulfide andmercaptans in water and acidic solutions are lower. Ammonium sulfatescrubbing media (including solutions and slurries, depending on solidscontent) accumulated in AS FGD systems after the absorption of sulfurdioxide from flue gases are acidic, with pH levels typically in a rangeof about 4 to about 6. When ammonia laden with hydrogen sulfide and/ormercaptans is introduced into the ammonium sulfate media, the ammonia isreadily absorbed and reacts with the absorbed sulfur dioxide, while thehydrogen sulfide and mercaptans are stripped from the liquid to the gasphase. In typical FGD systems, the reaction tank in which the scrubbingmedia is accumulated is often open at its upper end. Air that has beeninjected into the scrubbing media to react and form ammonium sulfatemixes with the stripped hydrogen sulfide and mercaptans, and theresulting mixture is then able to mix with the scrubbed flue gases,producing an undesirable odor and emission of harmful species.

The present invention reduces the release of hydrogen sulfide andmercaptans by adding the hydrogen sulfide and mercaptans-laden ammoniato the ammonium sulfate media in such a way that stripping of hydrogensulfide and mercaptans occurs in a confined zone, thus avoiding themixing of hydrogen sulfide and mercaptans gases with the scrubbed fluegas. The gaseous hydrogen sulfide and mercaptans are then diverted awayfrom the FGD system, and can be captured, incinerated, or otherwiseprevented from release into the atmosphere.

In accordance with the above, the AS FGD systems 10 and 100 representedin FIGS. 1 and 2 are each configured to separate hydrogen sulfide andmercaptans and prevent the contamination of a flue gas. The FGD systems10 and 100 are generally represented as being of a type that scrubs fluegases produced by the burning of fossil fuels or another process thatresults in the flue gas containing acidic gases, such as sulfur dioxide,hydrogen chloride and/or hydrogen fluoride, as well as particulatematter, nitrogen oxides (NOx), etc. Referring to FIG. 1, theconventional components of the FGD system 10 include an absorber tower14 having a contact zone 16 in which an ammonium sulfate scrubbing media(e.g., solution or slurry) 26 is brought into contact with a flue gasthat enters the FGD system 10 through an inlet duct 12. The scrubbingmedia 26 is shown as being collected in a tank 18 located at the bottomof the tower 14. The media 26 is drawn from the tank 18 with a pump 20and then delivered through a pipe 22 to the contact zone 16, where thescrubbing media 26 is dispersed with spray nozzles 24 or anothersuitable delivery device. After being scrubbed by the scrubbing media26, the resulting scrubbed stream of flue gas flows upward, typicallythrough a mist eliminator and/or other equipment (not shown), and iseventually released to atmosphere through a chimney or other suitablestructure. As with many existing wet flue gas desulfurizationfacilities, the FGD system 10 is equipped for in situ forced oxidationof the scrubbing media 26 that has collected in the tank 18. In FIG. 1,a suitable source 30 of oxygen, such as air or another oxygen-containinggas (hereinafter referred to as air for convenience), is represented asbeing introduced into the tank 18 with a sparger 28 connected to theoxygen source 30. The air is typically injected near the bottom of thetank 18, so that the gas migrates upward as bubbles 48 through thescrubbing media 26 in the tank 18. In this manner, reaction productsproduced by contacting the acidic gases of the flue gas with thescrubbing media 26 are oxidized by the oxygen in the sparged air topreferably yield ammonium sulfate as a useful fertilizer byproduct.

As taught by commonly-assigned U.S. Pat. Nos. 4,690,807 and 5,362,458,following absorption of the acidic gases present in a flue gas, theammonium sulfate media 26 collected in the tank 18 has a relatively lowpH, for example, about 4 to about 6. Aqueous ammonia (ammoniumhydroxide, NH₄OH) or another source of ammonia 31 is introduced into thecollected media 26, such as with a second sparger 32, typically near thetop of the reaction tank 18 and preferably a meter or so below thesurface of the scrubbing media 26 in the tank 18. The absorbed sulfurdioxide reacts with the ammonia to form ammonium sulfite (NH₄)₂SO₃ andammonium bisulfite (NH₄HSO₃), which are then oxidized in the presence ofsufficient oxygen (introduced by the sparger 28) to form precipitates ofammonium sulfate and ammonium bisulfate (NH₄HSO₄). Ammonium bisulfateundergoes a second reaction with the injected ammonia to form additionalammonium sulfate precipitate. A portion of the ammonium sulfate media 26is typically removed from the tank 18 and dewatered with a suitabledewatering device (not shown) to precipitate ammonium sulfate, which canthen be sold as a valuable fertilizer. If hydrogen chloride and hydrogenfluoride were present in the flue gas, as is typically the case withflue gas produced by the combustion of coal, these acidic gases are alsocaptured to form ammonium chloride and ammonium fluoride, which can beremoved in the same manner. Further details regarding thedesulfurization of flue gases can be obtained in the prior art,including the above-noted U.S. Pat. Nos. 4,690,807 and 5,362,458, andtherefore will not be discussed in any further detail here.

Any hydrogen sulfide and mercaptans present in the ammonia introducedwith the sparger 32 will also be present in the scrubbing media 26collected in the tank 18. A portion of the absorbed hydrogen sulfide andmercaptans is likely to be stripped from the scrubbing media 26 by theaction of the oxidation air introduced into the scrubbing media 26 bythe sparger 28, with the result that the oxidation air released from thesurface of the scrubbing media 26 in the tank 18 (as represented by thearrows in FIG. 1) will be accompanied by gaseous hydrogen sulfide andmercaptans. To inhibit mixing of the oxidation air and gaseous hydrogensulfide and mercaptans released from the scrubbing media 26 with theflue gas traveling through the contact zone 16, FIG. 1 shows a divider34 separating the reaction tank 18 from the contact zone 16. The divider34 is represented as having an inverted conical shape, with a passage 36located at its central lowermost extent through which the scrubbingmedia 26 can enter the tank 18 following contact with the flue gas inthe contact zone 16. The passage 36 is represented as being entirelyopen at its upper limit 38 and closed at its lower limit 40, which issubmerged below the surface of the scrubbing media 26 in the tank 18.The passage 36 further has one or more submerged peripheral openings 42located at its perimeter near its closed lower limit 40, and throughwhich the scrubbing media 26 is able to flow directly into the collectedmedia 26 within the tank 18.

The inverted conical shape of the divider 34 and the size andconfiguration of the passage 36 cause the oxidation air and gaseoushydrogen sulfide and mercaptans released from the scrubbing media 26 toflow toward the perimeter of the tank 18 above the scrubbing media 26,and into a confined zone 44 bounded by the surface of the scrubbingmedia 26, the divider 34, the exterior of the passage 36, and theexposed wall of the tank 18 between the surface of the scrubbing media26 and the divider 34. From the confined zone 44, the gaseous hydrogensulfide and mercaptans can be diverted through a pipe 46 or othersuitable structure and subsequently captured, incinerated, or otherwisedisposed of or processed in any desirable manner.

The FGD system 100 represented in FIG. 2 provides an alternativeconfiguration capable of removing hydrogen sulfide and mercaptansbrought into the system 100 with ammonia. Because of the similarities intheir construction and operation, consistent reference numbers are usedto identify functionally similar structures in FIGS. 1 and 2. The system100 of FIG. 2 primarily differs from that of FIG. 1 by providing adedicated hydrogen sulfide and mercaptans stripping vessel 50 in closeproximity to the tower 14, the latter of which again includes a contactzone 16 and reaction tank 18 whose purposes are essentially the same asdescribed for the embodiment of FIG. 1, including the collection of theammonium sulfate scrubbing media in the tank 18 to define a quantity ofcollected scrubbing media 26 a. The reaction tank 18 and the strippingvessel 50 are connected near their lower ends via a passage 62 so thatanother quantity of collected scrubbing media 26 b is within the vessel50. The passage 62 provides for equalization of the liquid levels of thecollected scrubbing media 26 a and 26 b within the tank 18 and vessel50.

As in the system 10 of FIG. 1, the system 100 of FIG. 2 utilizes a pump20 to draw the collected scrubbing media 26 a from the reaction tank 18,and then deliver the drawn scrubbing media 26 a through a pipe 22 tonozzles 24 located in the contact zone 16 of the tower 14. In addition,a portion of the drawn scrubbing media 26 a is shown as being deliveredby the pump 20 through a second pipe 52 to the stripping vessel 50,where the drawn scrubbing media 26 a is injected into the collectedscrubbing media 26 b within the vessel 50 through an inlet 61 located atsome distance below the surface of the scrubbing media 26 b in thevessel 50. For the reasons described in reference to the system 10 ofFIG. 1, air is preferably injected with a sparger 28 into the scrubbingmedia 26 a near the bottom of the reaction tank 18. Air is also injectedinto the collected scrubbing media 26 b within the stripping vessel 50with a sparger 58, preferably at roughly the same elevation as the airinjection site in the reaction tank 18. Instead of being injected intothe reaction tank 18 as done in FIG. 1, ammonia (and any hydrogensulfide and/or mercaptans contained therein) is drawn from a suitablesource 31 and injected with a sparger 32 into the stripping vessel 50.Similar to FIG. 1, the ammonia is preferably injected near the top ofthe vessel 50, for example, a meter or so below the surface of thescrubbing media 26 b in the vessel 50, and preferably below the locationof the inlet 61 where the scrubbing media 26 a is introduced into thescrubbing media 26 b within the vessel 50. Alternatively, the ammonialaden with hydrogen sulfide and mercaptans can be injected into the pipe52, so as to be mixed and delivered with the scrubbing media 26 aintroduced into the vessel 50.

The injected ammonia is captured by the low-pH scrubbing media 26 b asit flows downwardly through the stripping vessel 50 toward the passage62, which serves as an outlet of the vessel 50 to the tank 18. In sodoing, the scrubbing media 26 b within the vessel 50 flowscounter-currently to the air injected into the vessel 50 with thesparger 58. Simultaneously, the hydrogen sulfide and mercaptansintroduced with the ammonia into the vessel 50 are stripped out of thescrubbing media 26 b by the upward flow of bubbles 48 from the airsparger 58. Air and the stripped gaseous hydrogen sulfide and mercaptansexit the scrubbing media 26 b and enter the confined zone 44 between thesurface of the scrubbing media 26 b in the vessel 50 and the top of thevessel 50, and from there can be diverted through a pipe 46 or othersuitable structure and subsequently captured, incinerated, or otherwisedisposed of or processed in any desirable manner. The pH of thescrubbing media 26 b increases as it flows downward through thestripping vessel 50 as a result of the captured ammonia, and is returnedby gravity to the reaction tank 18 through the passage 62.

In view of the above, it can be appreciated that the systems 10 and 100effectively isolate and remove hydrogen sulfide and mercaptans that aretypically present in sources of ammonia (e.g., aqueous ammonia) used inAS FGD processes, and otherwise escape into the atmosphere with thescrubbed flue gases. As such, the invention avoids the disadvantages ofcurrent technologies that seek to avoid hydrogen sulfide and mercaptansby purifying ammonia using expensive and capital-intensive processes.

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the physical configuration of the systems 10 and 100could differ from that shown, and components other than those notedcould be used, including devices other than spargers and nozzles thatare adequately capable of dispensing fluids including air, ammonia, andscrubbing media in a suitable manner. Therefore, the scope of theinvention is to be limited only by the following claims.

The invention claimed is:
 1. A flue gas desulfurization systemcomprising: means for absorbing acidic gases from a flue gas bycontacting the flue gas with a scrubbing media containing ammoniumsulfate to produce a stream of scrubbed flue gas that is released toatmosphere; means for collecting the scrubbing media containing theabsorbed acidic gases, the collecting means comprising a tank into whichthe scrubbing media containing the absorbed acidic gases is receivedfrom the absorbing means to define a first quantity of collectedscrubbing media within the tank, the collecting means further comprisinga vessel fluidically coupled to the tank to receive the scrubbing mediafrom the first quantity of collected scrubbing media within tank andthereby define a second quantity of collected scrubbing media within thevessel, the vessel having an inlet below a surface of the secondquantity of collected scrubbing media through which the scrubbing mediais received from the tank and an outlet below the inlet that fluidicallyconnects the vessel to the tank so that the second quantity of collectedscrubbing media flows downwardly from the inlet to the outlet of thevessel and then flows into the first quantity of collected scrubbingmedia at a location below a surface of the first quantity of thecollected scrubbing media within the tank; a source of ammonia that isladen with at least one compound chosen from the group consisting ofhydrogen sulfide and mercaptans; means for injecting the ammonia fromthe source into the second quantity of collected scrubbing media betweenthe inlet and the outlet of the vessel, the injected ammonia beingabsorbed into and reacted with the second quantity of collectedscrubbing media within the vessel; means within the vessel for strippingthe at least one compound from the second quantity of collectedscrubbing media by causing the at least one compound to exit the secondquantity of collected scrubbing media as stripped gases, the strippingmeans comprising means for injecting an oxygen-containing gas into thesecond quantity of collected scrubbing media at a location above theoutlet of the vessel and below where the ammonia is injected from thesource into the second quantity of collected scrubbing media by themeans for injecting the ammonia; means for collecting the stripped gasesof the at least one compound without allowing the stripped gases toenter the stream of scrubbed flue gas; and means for isolating andremoving the stripped gases from the flue gas desulfurization system sothat the at least one compound contained in the stripped gases does notescape into the atmosphere with the stream of scrubbed flue gas.
 2. Theflue gas desulfurization system according to claim 1, further comprisingmeans for transporting the first quantity of collected scrubbing mediafrom the tank to the inlet of the vessel.
 3. The flue gasdesulfurization system according to claim 2, wherein the transportingmeans comprises a pump.
 4. The flue gas desulfurization system accordingto claim 1, wherein the vessel and the outlet thereof are configured sothat the second quantity of collected scrubbing media is returned bygravity through the outlet of the vessel to the first quantity ofcollected scrubbing media within the tank.
 5. The flue gasdesulfurization system according to claim 1, wherein the vessel, theoutlet thereof, and the means for injecting the oxygen-containing gasare configured so that the second quantity of collected scrubbing mediawithin the vessel flows to the outlet counter-currently to theoxygen-containing gas flowing through the second quantity of collectedscrubbing media and the at least one compound is stripped from thesecond quantity of collected scrubbing media by bubbles of theoxygen-containing gas that are formed by the means for injecting theoxygen-containing gas and rise through the second quantity of collectedscrubbing media within the vessel.
 6. The flue gas desulfurizationsystem according to claim 1, wherein the means for collecting thestripped gases collects the stripped gases in a confined zone between anupper end of the vessel and the surface of the second quantity ofcollected scrubbing media within the vessel.